1. Be as clear as possible for what you want to come of the campaign/adventure you're running. If it's completely open ended, say so. If you have a specific genre you'd like to push, say so. If you have a preference for how you'd like characters to be designed, say so and be both firm (in your resolve as to why) and somewhat flexible (as its also *their* game in some ways) with their choices. It is very important that players come to new campaigns with all sorts of baggage and ideas about how RPGing is to be done.
2. Be as clear as possible with your house rules, especially with those that affect character design choices (such as what spells you disallow or have altered).
3. Keep a notebook with you that you have labeled "Campaign Notes." Even if you don't have plans for what they are doing (trust me, it'll happen), glance at your "notes" from time to time to give yourself a moment to think. Also, if you're asked a question that you can't give a solid answer to (Are the hinges on this side of the door? Are the candles made from bees wax or some other type?) use a percentile die to answer for you.
4. Consider writing out or printing up statblocks of the monsters/creatures you intend to have them encounter. Nothing gets the players into "Gonna kill sumbody!" mode like seeing a DM pick up his or her Monster Manual as s/he asks the players to do Spot and Listen checks.
5. Think of balance whenever someone asks to do something that extends into nebulous rules areas or is nifty. On one hand, the goal is for players to have maximum fun per hour, but on the other hand you don't want to set a precedent where the monk can always jump flip kick off the PC dwarf's head to deal an extra 1d4 damage to opponents... because trust me, it'll happen almost every damn game, and you'll have the monk taking leadership just to put as many dwarves on the combat field as possible.)
6. Enjoy doing it, and enjoy the perks of being DM. Sit comfy in the best chair, and thank the players when they bring you soda. (note, these may just be regional perks- check your area for details).
